Especially when determining gaseous analytes (O2, CO2) in aqueous fluids problems may arise in sample measurement or calibration or quality control, if the sample or the calibrating or quality control medium does not completely fill the fluid carrying region of the sensoric element, or if gas bubbles, for instance air bubbles, are present in this region. Gas bubbles are likely to occur if the measurement channel of the sensor cartridge has non-uniform inner surfaces, which have differing wetting properties with fluids. Gas bubbles will most frequently arise or adhere at sites of the measurement channel where the wetting properties of the inner surfaces of the fluidic components or parts change discontinuously. This will for instance be the case when surfaces of different materials meet. The measurement channel usually comprises a multitude of fluidic parts made of different materials whose adjoining surfaces have different hydrophilic or hydrophobic qualities and thus different wetting properties.
U.S. Pat. No. 4,358,423 already mentions the problem of enclosed air bubbles which distort measurement results, since the bubbles impede sufficient wetting of the surface of the sensor element used. Measures for detecting such distortions must be taken especially in the case of automated analyzers where the filling process of the measurement capillary or the absence of bubbles in the measurement chamber must be monitored. The patent cited proposes to solve the problem by a method in which the electrical resistance between at least two points in the measurement chamber is measured and the filling process of the measurement chamber is controlled depending on the resistance value measured.
European Pat. No. 0 379 156 B1 describes coating methods, in which first a polyisocyanate solution is applied to the surface of a medical instrument (in particular a catheter), then the solution is (optionally) dried and subsequently a solution of a polymer containing carboxylic acid is applied. Such two- or multi-step methods with a plurality of agent solutions and chemical reactions usually require many process steps, which at the user site are not feasible at all or only at great cost.
Coating of surfaces of medical implants, catheters and pacemakers with layers containing chitosan is for instance known from U.S. Pat. No. 5,578,073, where it is used to reduce the risk of thrombosis when such devices of medical technology are introduced into the human body. The layer consists of chitosan and an additional, biologically active component, for instance PVA or serum albumin, which is embedded in a chitosan membrane. Such layers are not suitable for measurement channels with sensor elements, however.
In U.S. Pat. No. 4,752,426 there is described a method for hydrophilization of surfaces, in which chemically reactive groups or radicals are formed on the surface by means of a low-temperature plasma treatment. Subsequently a monomeric solution is applied to the surface. The monomers will chemically react with the chemically active groups or radicals on the surface, thus finally forming on the surface a coating by graft polymerization. This method has the disadvantage that the process steps must be carried out by the manufacturer and must be very precisely coordinated. The plasma treatment parameters for instance must be carefully chosen so that only such chemically reactive groups or radicals will be formed on the surface that can act as nuclei for subsequent graft polymerization.
In European Pat. No. 1 595 605 B1 the wetting problem is solved by providing a fluidic system (e.g., a sensor cartridge) for an analyzer, which comprises one or more fluidic parts (e.g., measurement channel) and at least one sensor element, where a film of hydrophilic polymer is applied on the inner surfaces of these parts without any intervening layers. The inner surface of the fluidic system is first given a physical-chemical pre-treatment. Subsequently the inner surfaces of the parts are brought into contact with a solution of the hydrophilic polymer, and then the solution is replaced by a gaseous medium, the surfaces remaining wetted by a part of the solution. Upon removal of the solvent a film of hydrophilic polymer is finally formed on the inner surfaces. This relatively complicated coating method can only be carried out at the manufacturing site.
From U.S. Pat. Appln. Pub. No. 2009/0130746 A1 there is known a factory-based method for coating the inner surfaces of a microchannel system, whose aim is to avoid non-specific adsorption of reagents in PCR-analysis. Here solutions containing chitosan or chitosan derivatives are used. The microchannel system of the chips is filled with a 5% solution of chitosan and covered by mineral oil. A 12 hour heat treatment at 75° C. follows. After removal of the chitosan solution the microchannels are flushed with a solvent, water and a buffer solution. The method is not suitable for measurement channels containing sensor elements.
In many applications it would, however, be of advantage if coating or hydrophilization of the inner surfaces of the measurement channel were not exclusively done at the manufacturing site, since hydrophilization should take place immediately before (or during) the actual use of the cartridge in an analyzer to avoid aging effects. If aqueous solutions are used for surface treatment at the manufacturing site, individual sensor elements might prematurely react with the aqueous solution and the sensor element might be activated by taking in water (“wet up” of the sensor)—an effect which would be undesirable.
It would also be conceivable to use a highly effective detergent in one of the operational fluids to obtain sufficient hydrophilization. Despite the undoubted efficacy of this measure, its use in the context of the present disclosure is not indicated due to undesirable side effects.